Generally, fluoroalkyl-group-containing surfactants, when made into aqueous solutions, are capable of greatly reducing surface tension, accordingly providing various properties such as emulsification, dispersion, solubilization, etc. However, they are not easily soluble in water.
Such surfactants are disclosed in, for example, Japanese Unexamined Patent Publications No. 2001-269564 and No. 1986-133244. These surfactants are, however, incapable of reducing surface tension to a satisfactory level, and since they are intended to be used in an organic solvent, they are not desirable in view of their detrimental effects on the environment and work safety.
Japanese Unexamined Patent Publication No. 1981-72071 discloses a case where in order to dissolve a fluorine-containing surfactant having low water solubility in water, the fluorine-containing surfactant must be used in combination with another surfactant.
Japanese Unexamined Patent Publication No. 1986-47948 discloses another kind of application of fluorine-containing surfactants. Such surfactants, however, are not easily soluble in water, so that they need to be used in a limited amount in an aqueous solution or to be used in combination with an organic solvent or other additives for dissolution.
In contrast, Japanese Unexamined Patent Publication No. 1982-164199 discloses a water-soluble fluorine-containing surfactant. The disclosed surfactant, however, dissolves into water as a salt, so that it tends to be influenced by the multicharged ions that are present in the system.
A surfactant containing a long chain fluoroalkyl group (having at least 8 carbon atoms), which has a high fluorine content, is advantageous for lowering surface tension, but it has low water solubility. Meanwhile, when the fluorine content of the surfactant is lowered in order to increase water solubility, surface tension cannot be lowered.